In recent years, awareness of environmental problems has been rising, and it has been pointed out that there is a possibility that halogen compounds, which have been widely used as flame retardants for plastic materials used in electrical or electronic apparatus products, might generate dioxins when the materials are incinerated for disposal. Because of this, there has been a shift to non-halogen flame retardants. Among these, as safe inorganic flame retardants having a large endothermic heat of decomposition, there is high demand for metal hydroxides, and aluminum hydroxide in particular, and the use thereof is further encouraged by the decomposition product being alumina, which is chemically stable. However, in recent years, there has been increasing concern about flame retarding materials inhibiting combustion when they are disposed of.
In order to solve the problem of global warming due to an increase in carbon dioxide, instead of plastic materials synthesized from fossil fuels, the utilization of a lactic acid polymer, which is synthesized from a renewable resource biomass and can be biorecycled and chemically recycled, is being actively developed. The advantage of using a lactic acid polymer is that, since the lactic acid polymer is synthesized from a biomass formed by fixing carbon dioxide, even if it is incinerated there is very little increase in carbon dioxide in the process overall, which supports the concept of carbon neutrality. Chemical recycling is a method in which an original starting material is regenerated using a small amount of energy, and is a method that is a step beyond the concept of carbon neutrality as an environmental countermeasure.
As a method for producing a lactic acid polymer, a technique of producing a lactic acid polymer by synthesizing lactide from a lactic acid oligomer by thermal decomposition and further polymerizing the lactide is well known in the art. In this production process, it is important to maintain optical purity. This is because a lactic acid polymer suitable for practical use is a transparent high rigidity polymer that is produced by ring-opening polymerization of an optically active L,L-lactide and has a melting point of about 175° C., and even a small decrease in the optical activity causes a large decrease in the melting point, resulting in loss of its utility.
A method in which, in order to impart flame retardancy to a lactic acid polymer synthesized from a renewable resource, a safe metal hydroxide such as aluminum hydroxide is used as a flame retardant, is already known from, for example, the publications below (ref. Patent Publications 1 to 3). However, the purpose of making the lactic acid polymer flame retardant is to suppress combustion or thermal decomposition thereof at high temperature, and a method in which the starting material lactide is recovered by thermal decomposition of a lactic acid polymer in the flame retardant composition is not disclosed in any of the publications.
Apart from making the lactic acid polymer flame retardant, the use of a metal compound as a catalyst when recovering the starting material lactide by thermal decomposition of the lactic acid polymer is also known. For example, with regard to thermal decomposition of a lactic acid polymer by an aluminum compound, Degee et al. have polymerized lactide using aluminum isopropoxide as an initiator and have further carried out thermal decomposition of the lactic acid polymer so produced (ref. Non-Patent Publication 1). Furthermore, Noda and Okuyama have carried out thermal decomposition of a lactic acid oligomer using aluminum isopropoxide and aluminum ethylacetoacetate, and have measured the purity of the lactide obtained as a decomposition product (ref. Non-Patent Publication 2). However, these techniques, which employ an aluminum compound, only disclose that the lactide thus recovered has very low optical purity.
Moreover, among patent publications, JP-A-6-279434 (Patent Publication 4) (JP-A denotes a Japanese unexamined patent application publication) discloses a technique for obtaining lactide having low optical purity (meso-isomer content 7-40%) from a lactic acid oligomer by the combined use of an alkali metal salt and a metal of group 4 to 15 in the periodic table and/or a salt thereof. The amount of a thermal decomposition catalyst added in this case is in the range of 0.01-5 parts by weight. JP-A-10-168077 (Patent Publication 5) and JP-A-10-306091 (Patent Publication 6) disclose, as thermal decomposition catalysts for a lactic acid oilgomer, metal compounds formed from group IIIA, group IVA, group IIB, group IVB, and group VA, and disclose, as aluminum compounds, aluminum ethoxide, aluminum isopropoxide, aluminum oxide, and aluminum chloride, which are compounds that do not function as flame retardants. JP-A-2000-15107 (Patent Publication 7) discloses a technique in which a special cyclic alkoxyaluminum compound is used as a thermal decomposition catalyst for a lactic acid oligomer at 0.1-1 mol % of the entire lactic acid units. None of these publications disclose a specific lactide recovery technique employing a metal hydroxide such as aluminum hydroxide.
As hereinbefore described, there is no technical disclosure in the art of a composition comprising a lactic acid polymer and a metal hydroxide such as aluminum hydroxide as a flame retardant, wherein recovery of lactide having high optical purity from the lactic acid polymer in the composition and the flame retardant are associated. This is because the concept of flame retardance, which is to suppress decomposition and combustion, and the concept of chemical recycling, which is to control the promotion of decomposition, are mutually contradictory. In addition, it has been shown in the art that, for example, recovery of lactide using an aluminum compound tends to degrade the optical purity. Under such circumstances, there has been a desire for a new technology that can achieve a balance between flame retardance and efficient recovery of lactide having high optical purity in order to apply a lactic acid polymer produced from a renewable resource to, for example, electrical and electronic apparatus components and carry out chemical recycling thereof economically.    (Patent Publication 1) JP-A-2001-303387    (Patent Publication 2) JP-A-2003-192925    (Patent Publication 3) JP-A-2004-75772    (Patent Publication 4) JP-A-6-279434    (Patent Publication 5) JP-A-10-168077    (Patent Publication 6) JP-A-10-306091    (Patent Publication 7) JP-A-2000-15107    (Non-Patent Publication 1) P. Degee et al., ‘Macromolecular Chemistry and Physics, Vol. 198, 1985 (1997)’    (Non-Patent Publication 2) Noda and Okuyama, ‘Shimadzu Hyouron (Shimadzu Review), Vol. 56, 169 (2000); M. Noda and H. Okuyama, Chemical & Pharmaceutical Bulletin, Vol. 47, 467 (1999)’